The present invention relates to an opto-electronic board and to a method of mounting an opto-electronic circuit on a hybrid printed circuit board. The hybrid printed circuit board comprises an optical waveguide and electrical tracks.
The transmission of high frequency signals over distances of 1 to 3 meters on printed wiring boards (PWB) and through several connectors, with low loss and distortion is desired in several applications such as high-speed servers and routers. As the signal frequencies are now in the 10 GHz range this is becoming increasingly difficult to accomplish electrically, mainly because of the dielectric losses of the insulating materials in the circuit board. Even if lower loss dielectric materials can be employed, as frequencies increase further, the resistance of the copper wiring of the PWB will eventually become a limiting factor.
One solution to this problem could be to transmit the higher frequency signals over the longer distances optically rather than electrically. This can be done by integrating optical waveguides in the printed wiring board. The waveguides carry optical signals in the plane of the PWB. The high frequency electrical signals are converted by means of opto-electronic converters into optical signals, coupled into a waveguide, integrated into the wiring board, transmitted to their destination (possibly also through several optical connectors) with low loss and distortion, coupled out of the waveguide and then converted back into electrical signals by means of further opto-electronic converters. The required mechanical tolerances of the placement of the emitters and detectors of the opto-electronic converters relative to the waveguide are of the order of a few micrometers, preferably <5 μm. This leads to the problem of mounting the opto-electronic converters relative to the waveguide so that the light can be efficiently coupled in and out of the waveguide.
In the patent application WO 2005/031417A1 a printed circuit board with electrical tracks and an integrated optical waveguide is described. An opto-electronic circuit is mounted on the surface of the printed circuit board and is optically coupled to the optical waveguide via an opening in the printed circuit board. Light is coupled out of the optical waveguide and directed to the opto-electronic circuit by turning round the light with 90° at the end face of the optical waveguide. Therefore, the end face of the optical waveguide is sloped.
In the publication R. T. Chen, L. Lin, C. Choi, Y. J. Liu, B. Bihari, L. Wu, S. Tang, R. Wickman, B. Picor, M. K. Hibbs-Brenner, J. Bristow, Y. S. Liu, “Fully Embedded Board-Level Guided-Wave Optoelectronic Interconnects,” Proc. IEEE, Vol. 88, No. 6, pp. 780-793, June. 2000, a printed circuit board is described with embedded optical waveguides and embedded optoelectronic elements. The optoelectronic elements are mounted on the surface of the optical waveguide layer. Mirrors embedded in the waveguide redirect the light to obtain efficient optical coupling between the optoelectronic elements and the optical waveguides. The realization of mirrors in the waveguide layer is a specialized process that increases the cost of the optical board.
In the publication G. K. Chang, D. Guidotti, Z. Huang, L. Wan, J. Yu, S. Hegde, H. F. Kuo, Y. J. Chang, F. Liu, F. Wang R. Tummala, “High-density, end-to-end optoelectronic integration and packaging for digital-optical interconnect systems,” Proc. SPIE Conf. on Enabling Photonics Technologies for Defense, Security and Aerospace Applications, 28 Mar.-01 Apr., 2005, Kissimmee, Fla., USA, Vol. 5814, (Paper 24), it is described how edge emitting lasers and edge receiving detectors are embedded in cavities in the waveguide layer to obtain a butt coupled optical interface to the waveguide facet. This approach avoids the use of mirrors but requires edge emitting lasers, which are generally more expensive than surface emitting lasers. Furthermore, edge receiving detectors are not available as standard components.
In the patent application WO 2005/096682 A2 the optoelectronic elements are embedded in the printed circuit board, where the surface emitting lasers and the detectors are oriented vertically. This arrangement also overcomes the requirement for waveguide embedded mirrors and enables a butt coupled optical interface to the waveguide end-facet. The electrical contacts to the optoelectronic element are obtained through microvias in the printed circuit board. Alternative options to establish the electrical connections between the printed circuit board and the optoelectronic element are by means of a flexible electrical sheet or by wire bonding. The precise positioning of the optoelectronic element with respect to the waveguides is obtained through mechanical alignment features in the board and on the optoelectronic element.